1. Field of the Invention
The present invention relates to an apparatus for transmitting/receiving a bit stream in a network and a method therefor, and more particularly, to an apparatus for transmitting/receiving a bit stream, by which a video bit stream is stably transferred in a network including a wireless network and an Internet network, and a method therefor. The present application is based on Korean Patent Application No. 00-9624 filed Feb. 26, 2000, which is incorporated herein by reference.
2. Description of the Related Art
In packet networks such as the Internet, typically, two logical ports exist when a channel is set to achieve communication between two locations. That is, a real time protocol (RTP) packet, which has been packetized according to a request for comments (RFC) protocol via the Internet network, is transmitted via the hierarchical structure of a real time protocol/user datagram protocol/Internet protocol (RTP/UDP/IP) or a real time protocol/transmission control protocol/Internet protocol (RTP/TCP/IP).
The RTP/TCP/IP operates in the acknowledge mode and can transmit data stably, so that it usually transmits control information. The RTP/UDP/IP operates in the unacknowledged mode, and transmits video data which usually must be processed in real time.
Referring to FIG. 1, in a transmitting terminal, a video bit stream sequentially undergoes an application layer (video source codec), an RTP layer, an UDP/IP and TCP/IP layer, a radio link protocol (RLP) layer, a layer 2 (L2) and a layer 1 (L1). In each layer, header information is added to the video bit stream, and the video bit stream having header information is transmitted to a network. Here, the TCP transmits control information, and the UDP transmits an RTP packet. In a receiving terminal, the video bit stream undergoes an UDP/IP and TCP/IP layer, an RTP layer and an application layer (video source codec), and is decoded into video data.
FIG. 2 is a view illustrating an embodiment of a typical communication method of transceiving data between a server and a client. First, a server 200 reads a video bit stream produced by a video source codec to divide the read video bit stream into a high priority bit stream 210 and a low priority bit stream 220 before an RTP packet is produced. Next, the server 200 transmits the high priority bit stream 210 to a client 230 via an RTP/TCP/IP, as indicated by arrow 1, and receives an acknowledgment representing that transmission of the high priority bit stream 210 has been completed without error, as indicated by arrow 2. Then the server 200 transmits the low priority bit stream 220 to the client 230 via an RTP/UDP/IP regardless of error as indicated by arrow 3. The client 230 reconstructs the high priority bit stream 210 and the low priority bit stream 220 back into the original video bit stream syntax.
As described above, under a conventional Internet environment, a bit stream produced by video source coding such as MPEG-4 or H.263 is transmitted to the UDP/IP layer. The UDP/IP layer cannot determine when all of the transmitted data has been received since it operates in an unacknowledged mode. If a bit stream packet is transmitted under a communication environment in which an Internet network communicates with a wireless network, the packet data may have bit error under the wireless environment. That is, the bit stream packet may have a loss on the Internet while undergoing the Internet environment and the wireless environment, and bit error may be generated even though the bit stream packet has been transmitted without loss. Here, the bit stream packet includes a packet header and a payload header. If a bit error is included in these headers, the receiving terminal cannot perform suitable decoding.
When a video bit stream is transmitted in real time, the transmitting terminal classifies the video bit stream based on priority, and transmits higher priority packets first and then lower priority packets. At this time, the receiving terminal must delay data until high priority packets are received first. Thus, real time processing of data is difficult. That is, if high priority packets are consecutively transmitted in an Internet network and a wireless network, the network must be continuously stable. Also, if a bit stream is classified according to priority during video source coding, it cannot be reconstructed into a bit stream packet which conforms to a current RFC protocol. Furthermore, the server 200 and the client 230 must always perform a pre-process for classifying a bit stream, according to priority, in order to transmit and receive the bit stream, and a post-process for reconstructing the received higher and lower priority bit streams into the original bit stream. Also, the process for classifying a bit stream based on priority before packetizing can only be performed at a video codec level which has already known the syntax of the video bit stream.
Therefore, data communication in an Internet network combined with a wireless environment causes packet loss and bit error as described above, thus deteriorating the quality of an image.